1. Field of the Invention
The invention relates to a nickel hydrogen rechargeable battery.
2. Description of the Related Art
Rare earth-Mg—Ni-based hydrogen storage alloy is capable of storing a large amount of hydrogen gas. The use of the rare earth-Mg—Ni-based hydrogen storage alloy in nickel hydrogen rechargeable batteries therefore contributes to the capacity increase of the batteries.
On the other hand, in spite of the high capacity, conventional nickel hydrogen rechargeable batteries using the rare earth-Mg—Ni-based hydrogen storage alloy need to be charged before use if they are stored for a long period of time because remaining capacity is decreased due to their high self-discharge rate. To improve the self-discharge characteristics, there has been considerable research on the batteries in which rare earth-Mg—Ni-based hydrogen storage alloy is used in their negative electrodes in the aim of increasing the capacity. As the result, a self-discharge restrained type of a nickel hydrogen rechargeable battery using the rare earth-Mg—Ni-based hydrogen storage alloy has been developed (see, for example, Unexamined Japanese Patent Publication (Kokai) No. 2007-149646).
As long as being charged beforehand, the battery of the self-discharge restrained type is reduced in remaining capacity at low reduction rate while being stored, so that they reduce the frequency of recharge required before use. With this advantage, the self-discharge restrained type of the nickel hydrogen rechargeable battery with the negative electrode including rare earth-Mg—Ni-based alloy is recognized as a battery that is excellent in terms both of having as good usability as a dry-cell battery and of having as high or higher capacity as a dry-cell battery.
Concerning the self-discharge restrained type of the nickel hydrogen rechargeable battery using the rare earth-Mg—Ni-based hydrogen storage alloy, the inventors examined the condition of the battery after storing the battery for a longer period of time than a conventionally supposed time period, and found that operating voltage was notably decreased in spite that sufficient capacity still remained. If the battery decreased in operating voltage as described is utilized in a device requiring relatively high operating voltage, it causes the problem that the device is unable to be activated even as there still remains battery capacity.